- Email: [email protected]
Surface modifications with heteropoly and isopoly oxometalates
207
J. Elecfroanal. Chem., 258 (1989) 207-218 Elsevier Sequoia S.A., Lausanne - Printed
Surface modifications
in The Netherlands
with heteropoly and isopoly oxometalates
Part IV *. Further details and kinetic aspects of the h.e.r. on the modified electrodes B. Keita and L. Nadjo ** D.C.P.R.,
U.A. 328, CNRS, ENSIC-INPL,
I rue Granduille, 54042 Nancy Cedex (France)
Roger Parsons University of Southampton, (Received
Department
of Chemistry
Southampton
SO9 5NH (Great Britain)
18 April 1988)
ABSTRACT Heteropoly and isopoly oxometalates can be used to modify electrode surfaces persistently, giving very durable and robust electrocatalytic materials for the h.e.r. Cyclic voltammetric characterization of the modified surfaces in pure acidic aqueous supporting electrolyte, reveals the presence of surface waves, which are pH-dependent and can interfere severely with the h.e.r. wave. Therefore, the kinetic analysis of the h.e.r. on the modified electrodes is performed by rotating disk voltammetry. The main Tafel parameters obtained for an electrode modified in the presence of ‘~z-K,P,W,,MoO,, are log (i,/A cm- 2)-- - 1 7 and n = 0.50, with a slope of 0.118 V and a stoichiometric number Y = 2. It appears that the mechanism of the h.e.r. on the electrode surfaces modified with heteropoly and isopoly oxometalates is different from that observed on platinum.
INTRODUCTION
In several reports, it has been demonstrated that heteropoly and isopoly oxometallates can be used to modify electrode surfaces persistently, giving very durable and robust modified materials [l-8]. In particular, upon modification, materials usually known to be poor surfaces from which to evolve hydrogen, show strikingly high activity towards this process. Part of these findings has been patented [9,10]. Furthermore, the modified surfaces show promise in accelerating the overall kinetics of oxygen reduction [5] and in changing the activity of glassy carbon towards the
For Parts I-III, see refs. 6-8. * To whom correspondence should be addressed. versite de Paris-Sud, Institut de Chimie Moltculaire l
l
Present address: Laboratoire d’Electrochimie, d’Orsay, BLtiment 420, 91405 Orsay, France.
Uni-
208
reduction of hexaammineruthenium(111) cation [7]. Usually, our test for “perfect” modification of the electrode surface is the h.e.r. In the present communication, further details are given on characteristics brought about by the modification in electrode properties. A kinetic study of the h.e.r. is also given. SUMMARY
OF PREVIOUS
OBSERVATIONS
Experimental conditions have been given previously [l-8]. Several results must be kept in mind. Although we have noticed that the speed of modification does clearly depend on the electrode material, the onset of the h.e.r. on thoroughly modified electrodes varies within a few millivolts for a given pH, and the corresponding exchange current densities are practically the same [l]. These observations would tend to support the idea that the prominent role in the electrocatalysis belongs to the electrodeposited compound. In the following, glassy carbon (GC, Tokai, Japan) electrodes are chosen as an example and used throughout. Among several favourable reasons, at least two advantages have dictated this choice: the material is compact, so that its geometric surface area is well-defined for use in semiquantitative and quantitative calculations; we also consider its possible industrial use in large scale devices, although graphite, which is also easy to derivatize [1,9,10], would appear more economic. We have demonstrated that the surface area of a glassy carbon electrode does not increase substantially upon modification with an HPA, and the geometric surface area could be used safely in current density calculations [8]. Furthermore, SEM microphotographs of the electrode have shown the spongy, uneven and discrete surface distribution of the catalyst [4]. Following the same reasoning as Kuwana [ll] or Saveant [12], it would appear that the real catalytic surface area could be less than the glassy carbon area. Thus, the reported current densities, which are calculated with respect to the geometric surface area of glassy carbon, appear as lower-limit values and the “true” current densities could be higher. RESULTS
Qualitative aspects It has been demonstrated previously [7] that ar,-P,W,,MoO,,K, (designated as P,W,,Mo in the following) in 0.5 M H,SO, at a glassy carbon electrode reveals the essential features displayed by numerous heteropoly and isopoly oxometalates in the same medium. Therefore, P,W,,Mo has been selected as an example throughout this work. However, some interesting details gathered during the study of other compounds will be stated briefly when useful. Derivatization of the GC electrode surface has been obtained as described previously [l-8]. For a typical experiment, a 8 x 10e4 M solution of P,W,,Mo was prepared in 0.5 M H,SO, and the potential of the working electrode was set to - 1.2 V vs. SCE in this solution under vigorous degassing and stirring with pure argon. In the experiment giving the modified electrode, the kinetic behaviour of which is studied hereafter, the completion of the
209
L
-0.3
#
I
0.0 POTENTIAL
I
I
to.3
I
6
+0.6
/ V vs. 5~
E
Fig. 1. Surface waves obtained in pure 0.5 M H2S04 with variously modified glassy carbon electrodes. (a) Glassy carbon electrode poised at -1.2 V vs. SCE in pure 0.5 M H,SO, for 27 h. Curve 1: cyclic voltammetry pattern after a few scans; curve 2: cyclic voltammetry pattern after the electrode has been exposed to air or cycled for a long time. (b) Glassy carbon electrode poised at - 1.2 V vs. SCE in 0.5 M H,S0,+8X10-4 M u~-P~W,~M~~~K~ and then used in pure 0.5 N H2S0,. This surface wave is stable. Electrode surface area: 0.07 cm2; scan rate: 100 mV/s.
modification has been considered satisfactory after 27 h. The electrode is then taken out of the derivatization medium, and rinsed thoroughly with 0.5 A4 H,SO,. This experiment has been complemented by a blank experiment in which a freshly polished GC electrode is treated in the same conditions but in pure 0.5 M H,SO, solution. In this case, compieteiy new electrodes and glassware are used. Figure 1 shows the main characteristics observed in pure 0.5 N H,SO,, for the treated electrodes. For the blank experiment, in line with the literature [13], curve 1 of Fig. la reveals mainly a surface redox couple. No attempt has been made to find out the
0.1
E/V
Ir
-1.B
:
:
: -1.2
:
;
; - 0.6
;
I
: 0.0
vs.SCE ;
:
: +06
Fig. 2. Cyclic vohammetry pattern for the h.e.r. in a pH = 2.94 solution, prepared as indicated in the text. . .) Freshly polished glassy carbon. () Glassy carbon modified in the presence of oaP,W,,MoO,,K, as indicated in the text. The surface wave resulting from the modification appears just positive of the h.e.r. wave. Electrode surface area: 0.07 cm’; scan rate: 100 mV/s.
(.
surface functionalities responsible for this wave. The important point, however, concerns the stability of this cyclic voltammetric pattern. After a few minutes of the electrode in air or after a few cyclic scans, curve 2 of Fig. la is obtained. On repeated scans, the wave corresponding to this surface redox couple decreases steadily, until it vanishes completely. It can be concluded that the new surface is neither stable to air, nor to repeated cycling. By contrast, Fig. lb shows the surface wave obtained persistently on electrodes modified in the presence of an oxometalate, in particular with P,W,,Mo. This wave is much larger than that of curve 1 of Fig. la and located at a distinctly different potential. The main electrochemical characteristics of this surface wave have been studied previously [7]. As an important detail, we emphasize the durability of this modified electrode which can be kept in the atmosphere of the laboratory and/or cycled for several weeks without any decay of the surface wave nor of the catalytic characteristics vis a vis the h.e.r. Clearly, it appears that modification in the presence of P,W,,Mo brings about the observed catalytic activity towards the h.e.r. In the following, the characterization of the heteropolyanion (HPA)-modified electrode will be performed in supporting electrolyte alone (x M H,SO, -t-y M K,SO,, x +y = 0.5), the total ionic strength being maintained constant. The complexity of the situation to be analyzed appears readily in Fig. 2. In cyclic voltammetry at a scan rate of 100 mV/s, Fig. 2 (dotted line) represents the hydrogen evolution wave at pH = 2.94 on a freshly polished glassy carbon electrode. In the same conditions, the solid line shows the result obtained after complete
211
modification of the surface. The whole voltammetric pattern moves positively and the surface redox couple wave due to derivatization in the presence of P,W,,Mo is not negligible compared to the h.e.r. wave. The striking observation, however, comes from the h.e.r. wave, which shows a substantial positive move, while its peak grows, sharpens and exhibits a reoxidation peak on potential reversal. This significant difference suggests that the kinetic parameters, in particular, (Y, change as the electrode is activated. The same trends are obtained during the electrocatalytic reduction of oxygen in acidic water on HPA-modified electrodes [5]. Similar observations have also been made [14] in the ascorbic acid oxidation on vacuum heat treated glassy carbon. The gradual modification of the electrode with an HPA, as already published for instance with SiW,,O,H, [l] also supports this interpretation. Taking into account the overlapping of curves suggested by Fig. 2, it appears that for very low pH values (pH G 1) part of the linear sweep voltammetry curve as well as the rotating disk voltammetry curve could be used safely for the kinetic analysis. However, as for intermediate pH values, such a procedure is not easily tractable, the rotating disk analysis with a rotation speed of 4500 rpm was preferred generally, and used in all the pH variation domain. Fig. 3 shows such a rotating disk voltammetry curve at pH = 2.94. By comparison with the solid line curve of Fig. lb, obtained at the same pH, it appears that rotating the electrode could help in minimizing substantially the interference of the surface wave. Conditions to obtain “perfectly” modified surfaces have been stated previously [6]. Kinetic analysis of the h.e.r. on the modified electrodes Classical Tafel analysis has been applied to the results like the one shown in Fig. 3. Mass transfer corrections have been taken into account when necessary. The various solutions are prepared just before the measurements and the pHs rechecked just after the experiments. For each series of experiments, the same modified electrode is used throughout, in order to avoid large surface area variations from one electrode to the other. Particular attention has been paid to work on “stabilized” electrodes only. We have observed that freshly modified electrodes can show, during the first scans in supporting electrolyte alone (for instance 0.5 M H,SO,), a strikingly large faradaic current at potentials substantially positive vs. SCE and therefore, well positive of the theoretical potential value of the h.e.r. This anomalous behaviour vanishes almost completely after several scans. No attempt to detect hydrogen in such experiments has been made. The possibilities that such a current be due to surface waves obtained in the absence and/or presence of P,W,,Mo, eventually mixed up with various proton interferences, have not been explored. Furthermore, reduced compounds in the heteropoly and isopoly oxometalate series which decompose water are known [15]. In addition, anomalous hydrogen evolution is not completely without precedent in the literature [16]. Taking into account the spongy nature of the present modified surface, as revealed by SEM microphotographs [4], a behaviour involving hydrogen can be explained as was done for platinized platinum electrodes [16], on the basis of absorption of hydrogen into the porous structure and its influence on the different stages of the h.e.r. After a few
I 100 pA
E/V”S.SCE 1
-0.9
I
I
-0.6
I
I
-0.3
I
I
0.0
Fig. 3. Rotating disc electrode voltammogram obtained in a pH = 2.94 solution for the hydrogen evolution reaction. The solution is thoroughly degassed with pure argon. Electrode surface area: 0.07 cm*. The electrode has been modified in 0.5 M H,SO, + 8 X 10m4 M a2-P,W,,MoO,,K, as described in the text. The potential scan rate is 10 mV/s. Rotation speed: 4500 rpm.
scans, however, this phenomenon corresponds to a negligible part of the current observed on the voltammetric pattern and will not be analyzed in detail here. Therefore, stabilized modified electrodes are those obtained after sufficiently numerous cyclic scans in supporting electrolyte alone. They are used throughout in this work. Under these conditions, we have demonstrated previously [7] that the theoretical wave corresponding to an infinitely fast reduction of the same amount of proton in solution at a bare electrode having the same surface area as the modified surface, shows, at any potential, a current higher than observed on the modified electrode. The experimental half-wave potential is less than 60 mV negative of the theoretical value. Figures 4 and 5 show illustrative Tafel lines, straight with little ambiguity. Figure 4 corresponds to glassy carbon treated in pure H,SO, and then used for the h.e.r. in a 0.5 M H,SO, solution. Its characteristics are gathered in Table 1 along with those for freshly polished glassy carbon. Figure 5 represents the h.e.r. kinetics on the HPA-modified electrode at pH = 2.94. In each case, the Tafel line is well defined. Tables 1 and 2 gather the main results pertaining to the P,W,,Mo modified surface. Except for “experimental” fluctuations, the results seem perfectly smooth
213
log
Ii/A
cr8(
-2.o_
-2.5
_
-3.0 _
-3.5 _
-100
-200
-300
Fig. 4. Typical Tafel line for the h.e.r. in a pH = 0.51 solution. The electrode is a glassy carbon electrode treated in pure 0.5 M H,SO,. For treatment conditions, see text. Linear sweep voltammetry; scan rate: 100 mV/s.
and show expected trends. Only the a value for pH = 2.94 is substantially different from 0.5. Here (r is the cathodic transfer coefficient as defined for multistep n-electron transfer processes, with possible chemical steps included. Table 2 also gathers the values of the ratio (n/v) where v is the stoichiometric number. These calculations were made by using the equation
TABLE
1
Tafel parameters of the h.e.r. on variously modified dures, see text. The test solution is 0.5 M H,SO, Treatment
medium
None (freshly polished glassy carbon) 0.5 M H, SO., 0.5 M H,SO, +8~10-~ M a,-P,W,,MoO,,K,
glassy
carbon
-log(i,/A 7.2 to 8.5 3.97 1.7
electrodes.
cm-*)
For modification
a 0.15 to 0.28 0.38 0.50
proce-
214
-0.8 pH
2.94
q
-0.4
0.0
,/’ I’ +0.4 ,/ I
/’ o
0
0
+ 0.8
q/mV 1
1 -40
1 -80
I -120
Fig. 5. Typical Tafel line for the h.e.r. in a pH = 2.94 solution. The electrode is a glassy carbon modified in the presence of orP,W,,MoO,rK, as described in the text. Rotating disc voltammetry; rotation speed: 4500 rpm.
electrode electrode
which is valid in the low-overpotential approximation regime. Although some scatter is observed, the average value of n/v is undoubtedly around 1.0. Exchange current density values are not unexpected, compared with literature determinations [ll] for glassy carbon electrodes modified with platinum microparticles dispersed in
TABLE
2
Tafel parameters for the h.e.r. on glassy carbon electrodes modified in the presence of a2-P,W,,Mo0,2K,. For modification procedure, see text. Test solutions contain a mixture of x M HaSO, + y M K,SO,, the ionic strength being maintained at p = 1.5 M PH
-log(i,/Acm-a)
slope V
a
n/v
0.51 1.86 2.94 3.87
1.7 2.06 2.25 2.80
0.118 0.123 0.087 0.105
0.50 0.48 0.68 0.56
1.17 0.85 1.36 0.83
a n/v-l.
a
215
0.1 5.06,;
E/V -0.50
-0.25
0.0
l0.25
vs. SCE +o.so
Fig. 6. Evolution, with the pH of the test soluton, of the surface wave which appears on a glassy carbon electrode modified in the presence of cr,-P,W,,MoO,,K, as descrbed in the text. The number on each curve indicates the pH of the test solution.
poly (Cvinylpyridine) film. Several points deserve notice: modification in plain H,SO,, although not stable, brings about a transitory enhancement of the h.e.r. kinetics as compared to the freshly polished glassy carbon. The stable modification with P,W,,Mo leads to i, values which compare advantageously with that of platinized platinum. However, before tackling the detailed discussion of the results, it is worth analyzing the voltammetric pattern of the persistent surface wave obtained on HPA-modified electrodes. Figure 6 represents part of the evolution with pH of this surface wave. Clearly, the electrochemical processes which take place are proton-consuming. For instance, the splitting of this wave starts from pH = 2.94 to higher values, but only the first of these split waves is represented on the figure. Therefore, this figure is mainly illustrative. We also emphasize that at higher pH values, the reduction of oxygen but mainly the oxidation of water commence on the modified electrode in the potential domain explored here. Although a comprehensive study of this surface wave is not available at present, it seems obvious that proton-consuming processes would interfere severely with the
216
h.e.r. in low proton concentration domains. It can therefore be anticipated that the results pertaining to such pH ranges will not be very easily analyzed. As a final remark, it is worth noticing that Au electrodes give results very similar to those of glassy carbon. In our hands, modified Au as utilized for the h.e.r. in 0.5 M H,SO,, is characterized by the following parameters: -log (i,/A cm-‘) - 2.5; slope = 111 mV; (Y= 0.54. It can be inferred that the activity of modified materials towards the h.e.r. does not depend on the nature of the conducting substrate [l], although no systematic attempt has been made to demonstrate this hypothesis, concerning particularly mechanistic aspects. DISCUSSION
P,W,,Mo has been chosen as a representative example. It must be emphasized that essentially the same trends and results are obtained with several other heteropoly and isopoly oxometalates described in preceding papers [l-lo]. Among several points to be stressed or discussed, the prominent role of the catalysts themselves must be recognized. It appears from results gathered in several papers [1,6] that the exchange current density does not depend on the nature of the conducting material of the electrode. Furthermore, gradual modification of the electrode surface [l] helps reinforce this idea. To elucidate this behaviour in more detail, work is in progress to entrap oxometalates in polymeric matrices and use such structures as catalysts [17,18]. Conducting polymers are scrutinized particularly in this respect. Turning now to the very nature of the catalysts themselves, it must be remembered that they are mainly inorganic polymers. This hypothesis is supported by the necessity of proton interference during their electrodeposition [8], which should create favourable conditions for polymerization [19]. The kinetic parameters in Tables 1 and 2 would suggest “classical” behaviour for the electrodes towards the h.e.r. Enhanced kinetics on passing from freshly polished glassy carbon to the modified electrodes is reflected in (-log i,,) values and concomitantly in (Yas appears in Table 2. Table 2 also shows expected trends, with the exchange current density decreasing from smaller to larger pH values. If an anodic transfer coefficient is defined as previously for CX,it is known that (Y+ p = n/v. Taking into account the values of o! and n/v in Table 2, it ensues that (Y+ p = 1 and j! = 0.5. A cross-checking of such a determination is useful and has been attempted through a quantitative handling of log i0 values. A plot of log i, against pH gives a line in the way one would expect, although the slope is only 0.31 with a correlation coefficient of 0.967. Another line going through the three lower pH points shows a good straight line of slope close to 0.20, the point at pH = 3.87 deviating considerably. However, this line will not be retained because the point at pH = 2.94 seems inadequate for the determination of the reaction order and always deviates largely from the good straight lines obtained at all overpotential values as stated in the following. The pH values have been measured carefully, and the reasons for such a discrepancy are not known at present. Turning back to the log i,
217
vs. pH plot, it appears that even discarding the point with OL= 0.68 results in a line with a slope of 0.33 and a correlation coefficient of 0.996. Clearly, the experimental results cannot be accommodated to a line with a slope of p = 1 - (Y.Undoubtedly, a direct observation and quantitative treatment of the anodic branch of the curve would have been useful. Unfortunately, several drawbacks preclude such an experiment. Among them, the potential location of the surface wave, its pH dependence and its current intensity distort any observation severely. Furthermore, hydrogen is poorly soluble in aqueous solutions and its oxidation, even in the absence of surface waves and in more favourable solvents, is difficult [20]. Therefore, it must be concluded that the discrepancies observed previously must be ascribed to uncertainties in the determination of exchange current densities. Several reasons can account for the difficulties in such measurements. For instance, it can be argued that the surface layer may be resistant and introduce errors in the measurements. Such an argument cannot hold, however, as it has been shown that the reduction kinetics of hexaammine ruthenium(II1) cation is enhanced on the modified glassy carbon and even approaches reversibility [7]. A more serious point concerns the possible interference of surface waves, essentially at intermediate pH values. They are proton-consuming, their potentials are pH-dependent and they cannot be considered negligible as compared to the remaining h.e.r. wave. Therefore, their presence may influence the analysis of the h.e.r. As a complementary point, several ion exchanger structures are known for instance in isopolyoxometalates series [15]. Also, the porous structure of the electrode surface has been revealed by SEM photomicrographs. Therefore, equilibration between the surface layer and the bulk of the solution may constitute another puzzle. Efforts to minimize or even suppress such interferences have been stressed previously. Finally, double layer effects have not been taken into account. The reaction order has been determined from the experimental results. For all the pH values quoted in Table 2, the current density at several overpotential values have been determined and the log i vs. log c u+ plots are drawn for each overpotential value. Nine overpotential values were selected and for all of them, very good straight lines are obtained through all the points, except for the point at pH = 2.94 which deviates largely. The average value for (a log i/a log c~+)~ is 0.46 + 0.03 which is fairly close to 0.5. To conclude, despite all the sources of inaccuracy in the present measurements, the values obtained for the various parameters are sufficiently reproducible and fairly close to “classical” parameters of the h.e.r., that they can be used tentatively to propound a global mechanism for this process on the present modified electrodes. With n/v = 1, the stoichiometric number v = 2 with n = 2. The cathodic transfer coefficient being 0.5, the main possibility left among h.e.r. mechanisms [21] is the one with a rate-determining discharge followed by chemical recombination. The result is different from the one obtained on clean platinum. From a practical point of view such a modified material which is very durable and robust, may replace platinum advantageously in many uses. As a fact, it behaves like a noble metal surface and we have shown that the reduction kinetics of
218
hexaammine ruthenium(II1) cation are enhanced substantially [7]. Also, the reduction of oxygen is strikingly catalyzed and the h.e.r. approaches sharply its thermodynamic reduction potential value [6,7]. Therefore, it seems that economic fuel cells may be constructed. In addition, the modified electrodes show promise in the oxidation of alcohols. One of the aims is to compare the activities of the present modified surfaces with those obtained by entrapping first the oxometalate in conducting and non-conducting polymeric matrices [17,18]. As a conclusion, work is in progress to establish how platinum-like are the surfaces modified with oxometalates and the possible improvements brought about by polymeric matrices. ACKNOWLEDGMENT
This work was supported by the CNRS (UA 328) and by the ENSIC-INPL. Dr. J.P. Ciabrini (Universite de Paris VII) is thanked for the gift of several pure samples of heteropoly oxometalates, including the one used in the present experiments. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
B. Keita and L. Nadjo, J. Electroanal. Chem., 191 (1985) 441. B. Keita and L. Nadjo, J. Electroanal. Chem., 199 (1986) 229. B. Keita, L. Nadjo, G. Krier and J.F. Muller, J. Electroanal. Chem., 223 (1987) 287. B. Keita, L. Nadjo and J.P. Haeussler, J. Electroanal. Chem., 230 (1987) 85. B. Keita and L. Nadjo, J. Electroanal. Chem., 227 (1987) 265. B. Keita and L. Nadjo, J. Electroanal. Chem., 243 (1988) 87. B. Keita, L. Nadjo and J.M. SavQnt, J. Electroanal. Chem., 243 (1988) 105. B. Keita and L. Nadjo, J. Electroanal. Chem., 247 (1988) 157. L. Nadjo and B. Keita, French Patent (CNRS), No. 8418094, 1984. L. Nadjo and B. Keita, European Patent (CNRS), No. 85402340.5, 1985. Extension to U.S.A., Japan, Canada, Norway, 1985. D.E. Bartak, B. Kazee, K. Shimazu and T. Kuwana, Anal. Chem., 58 (1986) 2756. C. Amatore, J.M. Savtant and D. Tessier, J. Electroanal. Chem., 147 (1983) 39. K. Shimazu, D. Weisshaar and T. Kuwana, J. Electroanal. Chem., 223 (1987) 223. D.T. Fagan, I.F. Hu and T. Kuwana, Anal. Chem., 57 (1985) 2759. C. Toume, Bull. Sot. Chim., 9 (1967) 3199. T. Kessler, A.M. Castro-Luna, W.E. Triaca and A.J. Arvia, J. Appl. Electrochem., 16 (1986) 693. B. Keita and L. Nadjo, J. Electroanal. Chem., 240 (1988) 325. B. Keita, J.P. Haeussler and L. Nadjo, J. Electroanal. Chem., 243 (1988) 481. J. Lemerle and J. Mandavo, New J. Chem., 11 (1987) 265. W.C. Barrette and D.T. Sawyer, Anal. Chem., 56 (1984) 653. J.D’M. Bockris and A.K.N. Reddy, Modem Electrochemistry, Vol. 2, Plenum Press, New York, 1970, pp. 1242-1250.
J. Elecfroanal. Chem., 258 (1989) 207-218 Elsevier Sequoia S.A., Lausanne - Printed
Surface modifications
in The Netherlands
with heteropoly and isopoly oxometalates
Part IV *. Further details and kinetic aspects of the h.e.r. on the modified electrodes B. Keita and L. Nadjo ** D.C.P.R.,
U.A. 328, CNRS, ENSIC-INPL,
I rue Granduille, 54042 Nancy Cedex (France)
Roger Parsons University of Southampton, (Received
Department
of Chemistry
Southampton
SO9 5NH (Great Britain)
18 April 1988)
ABSTRACT Heteropoly and isopoly oxometalates can be used to modify electrode surfaces persistently, giving very durable and robust electrocatalytic materials for the h.e.r. Cyclic voltammetric characterization of the modified surfaces in pure acidic aqueous supporting electrolyte, reveals the presence of surface waves, which are pH-dependent and can interfere severely with the h.e.r. wave. Therefore, the kinetic analysis of the h.e.r. on the modified electrodes is performed by rotating disk voltammetry. The main Tafel parameters obtained for an electrode modified in the presence of ‘~z-K,P,W,,MoO,, are log (i,/A cm- 2)-- - 1 7 and n = 0.50, with a slope of 0.118 V and a stoichiometric number Y = 2. It appears that the mechanism of the h.e.r. on the electrode surfaces modified with heteropoly and isopoly oxometalates is different from that observed on platinum.
INTRODUCTION
In several reports, it has been demonstrated that heteropoly and isopoly oxometallates can be used to modify electrode surfaces persistently, giving very durable and robust modified materials [l-8]. In particular, upon modification, materials usually known to be poor surfaces from which to evolve hydrogen, show strikingly high activity towards this process. Part of these findings has been patented [9,10]. Furthermore, the modified surfaces show promise in accelerating the overall kinetics of oxygen reduction [5] and in changing the activity of glassy carbon towards the
For Parts I-III, see refs. 6-8. * To whom correspondence should be addressed. versite de Paris-Sud, Institut de Chimie Moltculaire l
l
Present address: Laboratoire d’Electrochimie, d’Orsay, BLtiment 420, 91405 Orsay, France.
Uni-
208
reduction of hexaammineruthenium(111) cation [7]. Usually, our test for “perfect” modification of the electrode surface is the h.e.r. In the present communication, further details are given on characteristics brought about by the modification in electrode properties. A kinetic study of the h.e.r. is also given. SUMMARY
OF PREVIOUS
OBSERVATIONS
Experimental conditions have been given previously [l-8]. Several results must be kept in mind. Although we have noticed that the speed of modification does clearly depend on the electrode material, the onset of the h.e.r. on thoroughly modified electrodes varies within a few millivolts for a given pH, and the corresponding exchange current densities are practically the same [l]. These observations would tend to support the idea that the prominent role in the electrocatalysis belongs to the electrodeposited compound. In the following, glassy carbon (GC, Tokai, Japan) electrodes are chosen as an example and used throughout. Among several favourable reasons, at least two advantages have dictated this choice: the material is compact, so that its geometric surface area is well-defined for use in semiquantitative and quantitative calculations; we also consider its possible industrial use in large scale devices, although graphite, which is also easy to derivatize [1,9,10], would appear more economic. We have demonstrated that the surface area of a glassy carbon electrode does not increase substantially upon modification with an HPA, and the geometric surface area could be used safely in current density calculations [8]. Furthermore, SEM microphotographs of the electrode have shown the spongy, uneven and discrete surface distribution of the catalyst [4]. Following the same reasoning as Kuwana [ll] or Saveant [12], it would appear that the real catalytic surface area could be less than the glassy carbon area. Thus, the reported current densities, which are calculated with respect to the geometric surface area of glassy carbon, appear as lower-limit values and the “true” current densities could be higher. RESULTS
Qualitative aspects It has been demonstrated previously [7] that ar,-P,W,,MoO,,K, (designated as P,W,,Mo in the following) in 0.5 M H,SO, at a glassy carbon electrode reveals the essential features displayed by numerous heteropoly and isopoly oxometalates in the same medium. Therefore, P,W,,Mo has been selected as an example throughout this work. However, some interesting details gathered during the study of other compounds will be stated briefly when useful. Derivatization of the GC electrode surface has been obtained as described previously [l-8]. For a typical experiment, a 8 x 10e4 M solution of P,W,,Mo was prepared in 0.5 M H,SO, and the potential of the working electrode was set to - 1.2 V vs. SCE in this solution under vigorous degassing and stirring with pure argon. In the experiment giving the modified electrode, the kinetic behaviour of which is studied hereafter, the completion of the
209
L
-0.3
#
I
0.0 POTENTIAL
I
I
to.3
I
6
+0.6
/ V vs. 5~
E
Fig. 1. Surface waves obtained in pure 0.5 M H2S04 with variously modified glassy carbon electrodes. (a) Glassy carbon electrode poised at -1.2 V vs. SCE in pure 0.5 M H,SO, for 27 h. Curve 1: cyclic voltammetry pattern after a few scans; curve 2: cyclic voltammetry pattern after the electrode has been exposed to air or cycled for a long time. (b) Glassy carbon electrode poised at - 1.2 V vs. SCE in 0.5 M H,S0,+8X10-4 M u~-P~W,~M~~~K~ and then used in pure 0.5 N H2S0,. This surface wave is stable. Electrode surface area: 0.07 cm2; scan rate: 100 mV/s.
modification has been considered satisfactory after 27 h. The electrode is then taken out of the derivatization medium, and rinsed thoroughly with 0.5 A4 H,SO,. This experiment has been complemented by a blank experiment in which a freshly polished GC electrode is treated in the same conditions but in pure 0.5 M H,SO, solution. In this case, compieteiy new electrodes and glassware are used. Figure 1 shows the main characteristics observed in pure 0.5 N H,SO,, for the treated electrodes. For the blank experiment, in line with the literature [13], curve 1 of Fig. la reveals mainly a surface redox couple. No attempt has been made to find out the
0.1
E/V
Ir
-1.B
:
:
: -1.2
:
;
; - 0.6
;
I
: 0.0
vs.SCE ;
:
: +06
Fig. 2. Cyclic vohammetry pattern for the h.e.r. in a pH = 2.94 solution, prepared as indicated in the text. . .) Freshly polished glassy carbon. () Glassy carbon modified in the presence of oaP,W,,MoO,,K, as indicated in the text. The surface wave resulting from the modification appears just positive of the h.e.r. wave. Electrode surface area: 0.07 cm’; scan rate: 100 mV/s.
(.
surface functionalities responsible for this wave. The important point, however, concerns the stability of this cyclic voltammetric pattern. After a few minutes of the electrode in air or after a few cyclic scans, curve 2 of Fig. la is obtained. On repeated scans, the wave corresponding to this surface redox couple decreases steadily, until it vanishes completely. It can be concluded that the new surface is neither stable to air, nor to repeated cycling. By contrast, Fig. lb shows the surface wave obtained persistently on electrodes modified in the presence of an oxometalate, in particular with P,W,,Mo. This wave is much larger than that of curve 1 of Fig. la and located at a distinctly different potential. The main electrochemical characteristics of this surface wave have been studied previously [7]. As an important detail, we emphasize the durability of this modified electrode which can be kept in the atmosphere of the laboratory and/or cycled for several weeks without any decay of the surface wave nor of the catalytic characteristics vis a vis the h.e.r. Clearly, it appears that modification in the presence of P,W,,Mo brings about the observed catalytic activity towards the h.e.r. In the following, the characterization of the heteropolyanion (HPA)-modified electrode will be performed in supporting electrolyte alone (x M H,SO, -t-y M K,SO,, x +y = 0.5), the total ionic strength being maintained constant. The complexity of the situation to be analyzed appears readily in Fig. 2. In cyclic voltammetry at a scan rate of 100 mV/s, Fig. 2 (dotted line) represents the hydrogen evolution wave at pH = 2.94 on a freshly polished glassy carbon electrode. In the same conditions, the solid line shows the result obtained after complete
211
modification of the surface. The whole voltammetric pattern moves positively and the surface redox couple wave due to derivatization in the presence of P,W,,Mo is not negligible compared to the h.e.r. wave. The striking observation, however, comes from the h.e.r. wave, which shows a substantial positive move, while its peak grows, sharpens and exhibits a reoxidation peak on potential reversal. This significant difference suggests that the kinetic parameters, in particular, (Y, change as the electrode is activated. The same trends are obtained during the electrocatalytic reduction of oxygen in acidic water on HPA-modified electrodes [5]. Similar observations have also been made [14] in the ascorbic acid oxidation on vacuum heat treated glassy carbon. The gradual modification of the electrode with an HPA, as already published for instance with SiW,,O,H, [l] also supports this interpretation. Taking into account the overlapping of curves suggested by Fig. 2, it appears that for very low pH values (pH G 1) part of the linear sweep voltammetry curve as well as the rotating disk voltammetry curve could be used safely for the kinetic analysis. However, as for intermediate pH values, such a procedure is not easily tractable, the rotating disk analysis with a rotation speed of 4500 rpm was preferred generally, and used in all the pH variation domain. Fig. 3 shows such a rotating disk voltammetry curve at pH = 2.94. By comparison with the solid line curve of Fig. lb, obtained at the same pH, it appears that rotating the electrode could help in minimizing substantially the interference of the surface wave. Conditions to obtain “perfectly” modified surfaces have been stated previously [6]. Kinetic analysis of the h.e.r. on the modified electrodes Classical Tafel analysis has been applied to the results like the one shown in Fig. 3. Mass transfer corrections have been taken into account when necessary. The various solutions are prepared just before the measurements and the pHs rechecked just after the experiments. For each series of experiments, the same modified electrode is used throughout, in order to avoid large surface area variations from one electrode to the other. Particular attention has been paid to work on “stabilized” electrodes only. We have observed that freshly modified electrodes can show, during the first scans in supporting electrolyte alone (for instance 0.5 M H,SO,), a strikingly large faradaic current at potentials substantially positive vs. SCE and therefore, well positive of the theoretical potential value of the h.e.r. This anomalous behaviour vanishes almost completely after several scans. No attempt to detect hydrogen in such experiments has been made. The possibilities that such a current be due to surface waves obtained in the absence and/or presence of P,W,,Mo, eventually mixed up with various proton interferences, have not been explored. Furthermore, reduced compounds in the heteropoly and isopoly oxometalate series which decompose water are known [15]. In addition, anomalous hydrogen evolution is not completely without precedent in the literature [16]. Taking into account the spongy nature of the present modified surface, as revealed by SEM microphotographs [4], a behaviour involving hydrogen can be explained as was done for platinized platinum electrodes [16], on the basis of absorption of hydrogen into the porous structure and its influence on the different stages of the h.e.r. After a few
I 100 pA
E/V”S.SCE 1
-0.9
I
I
-0.6
I
I
-0.3
I
I
0.0
Fig. 3. Rotating disc electrode voltammogram obtained in a pH = 2.94 solution for the hydrogen evolution reaction. The solution is thoroughly degassed with pure argon. Electrode surface area: 0.07 cm*. The electrode has been modified in 0.5 M H,SO, + 8 X 10m4 M a2-P,W,,MoO,,K, as described in the text. The potential scan rate is 10 mV/s. Rotation speed: 4500 rpm.
scans, however, this phenomenon corresponds to a negligible part of the current observed on the voltammetric pattern and will not be analyzed in detail here. Therefore, stabilized modified electrodes are those obtained after sufficiently numerous cyclic scans in supporting electrolyte alone. They are used throughout in this work. Under these conditions, we have demonstrated previously [7] that the theoretical wave corresponding to an infinitely fast reduction of the same amount of proton in solution at a bare electrode having the same surface area as the modified surface, shows, at any potential, a current higher than observed on the modified electrode. The experimental half-wave potential is less than 60 mV negative of the theoretical value. Figures 4 and 5 show illustrative Tafel lines, straight with little ambiguity. Figure 4 corresponds to glassy carbon treated in pure H,SO, and then used for the h.e.r. in a 0.5 M H,SO, solution. Its characteristics are gathered in Table 1 along with those for freshly polished glassy carbon. Figure 5 represents the h.e.r. kinetics on the HPA-modified electrode at pH = 2.94. In each case, the Tafel line is well defined. Tables 1 and 2 gather the main results pertaining to the P,W,,Mo modified surface. Except for “experimental” fluctuations, the results seem perfectly smooth
213
log
Ii/A
cr8(
-2.o_
-2.5
_
-3.0 _
-3.5 _
-100
-200
-300
Fig. 4. Typical Tafel line for the h.e.r. in a pH = 0.51 solution. The electrode is a glassy carbon electrode treated in pure 0.5 M H,SO,. For treatment conditions, see text. Linear sweep voltammetry; scan rate: 100 mV/s.
and show expected trends. Only the a value for pH = 2.94 is substantially different from 0.5. Here (r is the cathodic transfer coefficient as defined for multistep n-electron transfer processes, with possible chemical steps included. Table 2 also gathers the values of the ratio (n/v) where v is the stoichiometric number. These calculations were made by using the equation
TABLE
1
Tafel parameters of the h.e.r. on variously modified dures, see text. The test solution is 0.5 M H,SO, Treatment
medium
None (freshly polished glassy carbon) 0.5 M H, SO., 0.5 M H,SO, +8~10-~ M a,-P,W,,MoO,,K,
glassy
carbon
-log(i,/A 7.2 to 8.5 3.97 1.7
electrodes.
cm-*)
For modification
a 0.15 to 0.28 0.38 0.50
proce-
214
-0.8 pH
2.94
q
-0.4
0.0
,/’ I’ +0.4 ,/ I
/’ o
0
0
+ 0.8
q/mV 1
1 -40
1 -80
I -120
Fig. 5. Typical Tafel line for the h.e.r. in a pH = 2.94 solution. The electrode is a glassy carbon modified in the presence of orP,W,,MoO,rK, as described in the text. Rotating disc voltammetry; rotation speed: 4500 rpm.
electrode electrode
which is valid in the low-overpotential approximation regime. Although some scatter is observed, the average value of n/v is undoubtedly around 1.0. Exchange current density values are not unexpected, compared with literature determinations [ll] for glassy carbon electrodes modified with platinum microparticles dispersed in
TABLE
2
Tafel parameters for the h.e.r. on glassy carbon electrodes modified in the presence of a2-P,W,,Mo0,2K,. For modification procedure, see text. Test solutions contain a mixture of x M HaSO, + y M K,SO,, the ionic strength being maintained at p = 1.5 M PH
-log(i,/Acm-a)
slope V
a
n/v
0.51 1.86 2.94 3.87
1.7 2.06 2.25 2.80
0.118 0.123 0.087 0.105
0.50 0.48 0.68 0.56
1.17 0.85 1.36 0.83
a n/v-l.
a
215
0.1 5.06,;
E/V -0.50
-0.25
0.0
l0.25
vs. SCE +o.so
Fig. 6. Evolution, with the pH of the test soluton, of the surface wave which appears on a glassy carbon electrode modified in the presence of cr,-P,W,,MoO,,K, as descrbed in the text. The number on each curve indicates the pH of the test solution.
poly (Cvinylpyridine) film. Several points deserve notice: modification in plain H,SO,, although not stable, brings about a transitory enhancement of the h.e.r. kinetics as compared to the freshly polished glassy carbon. The stable modification with P,W,,Mo leads to i, values which compare advantageously with that of platinized platinum. However, before tackling the detailed discussion of the results, it is worth analyzing the voltammetric pattern of the persistent surface wave obtained on HPA-modified electrodes. Figure 6 represents part of the evolution with pH of this surface wave. Clearly, the electrochemical processes which take place are proton-consuming. For instance, the splitting of this wave starts from pH = 2.94 to higher values, but only the first of these split waves is represented on the figure. Therefore, this figure is mainly illustrative. We also emphasize that at higher pH values, the reduction of oxygen but mainly the oxidation of water commence on the modified electrode in the potential domain explored here. Although a comprehensive study of this surface wave is not available at present, it seems obvious that proton-consuming processes would interfere severely with the
216
h.e.r. in low proton concentration domains. It can therefore be anticipated that the results pertaining to such pH ranges will not be very easily analyzed. As a final remark, it is worth noticing that Au electrodes give results very similar to those of glassy carbon. In our hands, modified Au as utilized for the h.e.r. in 0.5 M H,SO,, is characterized by the following parameters: -log (i,/A cm-‘) - 2.5; slope = 111 mV; (Y= 0.54. It can be inferred that the activity of modified materials towards the h.e.r. does not depend on the nature of the conducting substrate [l], although no systematic attempt has been made to demonstrate this hypothesis, concerning particularly mechanistic aspects. DISCUSSION
P,W,,Mo has been chosen as a representative example. It must be emphasized that essentially the same trends and results are obtained with several other heteropoly and isopoly oxometalates described in preceding papers [l-lo]. Among several points to be stressed or discussed, the prominent role of the catalysts themselves must be recognized. It appears from results gathered in several papers [1,6] that the exchange current density does not depend on the nature of the conducting material of the electrode. Furthermore, gradual modification of the electrode surface [l] helps reinforce this idea. To elucidate this behaviour in more detail, work is in progress to entrap oxometalates in polymeric matrices and use such structures as catalysts [17,18]. Conducting polymers are scrutinized particularly in this respect. Turning now to the very nature of the catalysts themselves, it must be remembered that they are mainly inorganic polymers. This hypothesis is supported by the necessity of proton interference during their electrodeposition [8], which should create favourable conditions for polymerization [19]. The kinetic parameters in Tables 1 and 2 would suggest “classical” behaviour for the electrodes towards the h.e.r. Enhanced kinetics on passing from freshly polished glassy carbon to the modified electrodes is reflected in (-log i,,) values and concomitantly in (Yas appears in Table 2. Table 2 also shows expected trends, with the exchange current density decreasing from smaller to larger pH values. If an anodic transfer coefficient is defined as previously for CX,it is known that (Y+ p = n/v. Taking into account the values of o! and n/v in Table 2, it ensues that (Y+ p = 1 and j! = 0.5. A cross-checking of such a determination is useful and has been attempted through a quantitative handling of log i0 values. A plot of log i, against pH gives a line in the way one would expect, although the slope is only 0.31 with a correlation coefficient of 0.967. Another line going through the three lower pH points shows a good straight line of slope close to 0.20, the point at pH = 3.87 deviating considerably. However, this line will not be retained because the point at pH = 2.94 seems inadequate for the determination of the reaction order and always deviates largely from the good straight lines obtained at all overpotential values as stated in the following. The pH values have been measured carefully, and the reasons for such a discrepancy are not known at present. Turning back to the log i,
217
vs. pH plot, it appears that even discarding the point with OL= 0.68 results in a line with a slope of 0.33 and a correlation coefficient of 0.996. Clearly, the experimental results cannot be accommodated to a line with a slope of p = 1 - (Y.Undoubtedly, a direct observation and quantitative treatment of the anodic branch of the curve would have been useful. Unfortunately, several drawbacks preclude such an experiment. Among them, the potential location of the surface wave, its pH dependence and its current intensity distort any observation severely. Furthermore, hydrogen is poorly soluble in aqueous solutions and its oxidation, even in the absence of surface waves and in more favourable solvents, is difficult [20]. Therefore, it must be concluded that the discrepancies observed previously must be ascribed to uncertainties in the determination of exchange current densities. Several reasons can account for the difficulties in such measurements. For instance, it can be argued that the surface layer may be resistant and introduce errors in the measurements. Such an argument cannot hold, however, as it has been shown that the reduction kinetics of hexaammine ruthenium(II1) cation is enhanced on the modified glassy carbon and even approaches reversibility [7]. A more serious point concerns the possible interference of surface waves, essentially at intermediate pH values. They are proton-consuming, their potentials are pH-dependent and they cannot be considered negligible as compared to the remaining h.e.r. wave. Therefore, their presence may influence the analysis of the h.e.r. As a complementary point, several ion exchanger structures are known for instance in isopolyoxometalates series [15]. Also, the porous structure of the electrode surface has been revealed by SEM photomicrographs. Therefore, equilibration between the surface layer and the bulk of the solution may constitute another puzzle. Efforts to minimize or even suppress such interferences have been stressed previously. Finally, double layer effects have not been taken into account. The reaction order has been determined from the experimental results. For all the pH values quoted in Table 2, the current density at several overpotential values have been determined and the log i vs. log c u+ plots are drawn for each overpotential value. Nine overpotential values were selected and for all of them, very good straight lines are obtained through all the points, except for the point at pH = 2.94 which deviates largely. The average value for (a log i/a log c~+)~ is 0.46 + 0.03 which is fairly close to 0.5. To conclude, despite all the sources of inaccuracy in the present measurements, the values obtained for the various parameters are sufficiently reproducible and fairly close to “classical” parameters of the h.e.r., that they can be used tentatively to propound a global mechanism for this process on the present modified electrodes. With n/v = 1, the stoichiometric number v = 2 with n = 2. The cathodic transfer coefficient being 0.5, the main possibility left among h.e.r. mechanisms [21] is the one with a rate-determining discharge followed by chemical recombination. The result is different from the one obtained on clean platinum. From a practical point of view such a modified material which is very durable and robust, may replace platinum advantageously in many uses. As a fact, it behaves like a noble metal surface and we have shown that the reduction kinetics of
218
hexaammine ruthenium(II1) cation are enhanced substantially [7]. Also, the reduction of oxygen is strikingly catalyzed and the h.e.r. approaches sharply its thermodynamic reduction potential value [6,7]. Therefore, it seems that economic fuel cells may be constructed. In addition, the modified electrodes show promise in the oxidation of alcohols. One of the aims is to compare the activities of the present modified surfaces with those obtained by entrapping first the oxometalate in conducting and non-conducting polymeric matrices [17,18]. As a conclusion, work is in progress to establish how platinum-like are the surfaces modified with oxometalates and the possible improvements brought about by polymeric matrices. ACKNOWLEDGMENT
This work was supported by the CNRS (UA 328) and by the ENSIC-INPL. Dr. J.P. Ciabrini (Universite de Paris VII) is thanked for the gift of several pure samples of heteropoly oxometalates, including the one used in the present experiments. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
B. Keita and L. Nadjo, J. Electroanal. Chem., 191 (1985) 441. B. Keita and L. Nadjo, J. Electroanal. Chem., 199 (1986) 229. B. Keita, L. Nadjo, G. Krier and J.F. Muller, J. Electroanal. Chem., 223 (1987) 287. B. Keita, L. Nadjo and J.P. Haeussler, J. Electroanal. Chem., 230 (1987) 85. B. Keita and L. Nadjo, J. Electroanal. Chem., 227 (1987) 265. B. Keita and L. Nadjo, J. Electroanal. Chem., 243 (1988) 87. B. Keita, L. Nadjo and J.M. SavQnt, J. Electroanal. Chem., 243 (1988) 105. B. Keita and L. Nadjo, J. Electroanal. Chem., 247 (1988) 157. L. Nadjo and B. Keita, French Patent (CNRS), No. 8418094, 1984. L. Nadjo and B. Keita, European Patent (CNRS), No. 85402340.5, 1985. Extension to U.S.A., Japan, Canada, Norway, 1985. D.E. Bartak, B. Kazee, K. Shimazu and T. Kuwana, Anal. Chem., 58 (1986) 2756. C. Amatore, J.M. Savtant and D. Tessier, J. Electroanal. Chem., 147 (1983) 39. K. Shimazu, D. Weisshaar and T. Kuwana, J. Electroanal. Chem., 223 (1987) 223. D.T. Fagan, I.F. Hu and T. Kuwana, Anal. Chem., 57 (1985) 2759. C. Toume, Bull. Sot. Chim., 9 (1967) 3199. T. Kessler, A.M. Castro-Luna, W.E. Triaca and A.J. Arvia, J. Appl. Electrochem., 16 (1986) 693. B. Keita and L. Nadjo, J. Electroanal. Chem., 240 (1988) 325. B. Keita, J.P. Haeussler and L. Nadjo, J. Electroanal. Chem., 243 (1988) 481. J. Lemerle and J. Mandavo, New J. Chem., 11 (1987) 265. W.C. Barrette and D.T. Sawyer, Anal. Chem., 56 (1984) 653. J.D’M. Bockris and A.K.N. Reddy, Modem Electrochemistry, Vol. 2, Plenum Press, New York, 1970, pp. 1242-1250.